Carrier wave telephony



Sept. 24, 1940. T. s. sKxLLMAN CARRIER WAVE TELEPHONY Filed nec. 17, 1958 atented Sept. 24,1940

PATENT :OFFICE cARRIER. WAVE TELEPHONY Thomas Samuel Skillman, Eindhoven, 'Nether` lands, assignonby mesne assignments, to Hartford National Bank and Trust Company, Hartford, Conn., as .trustee VApplication December 17, 1938, Serial No. 246,458 A In ny December 27, 1937 My invention relates to a signal receiver for use in carrier wave telephony systems. More particu-l larly. my invention is concerned with systems in which signals, such as call currents or selective impulses, are sent from one station to another by VItransmitting a'carrier wave which is suppressed during the transmission of conversation, or by a signal current which is included in the conversation side-band.

In such systems the signal is sifted out by including in the corresponding signal receiver, a iilter or an oscillating circuit which is tuned to the carrier-wave frequency or to the frequency of the signal current included in the conversation side-band. The signal current which is separated by the above means controls, through a rectifier, a suitable relay which in turn controls the signal circuit.

I have found that in such systems there is danger that false responses may be caused in the signal circuit by oscillations which occur during the` transmission of the conversation and whose frequency is the same or close to that of the signal current.

The object of my invention is to overcome the above diiliculty, and for this purpose I provide amplitude-limiting means in the input circuit of the signal receiver. ,y

These means, which preferably comprises a grid-controlled electron tube whose grid circuit includes a condenser and a.v leakage resistance, has the further advantage that it allows a smaller amplitude difference between the amplitude of the signal current and the mean amplitude of the conversation side-band.

In order that the invention may be clearly understood and readily carried into effect, I shall describe the same in more detail with reference to the accompanying drawing, in which the single gure is a schematic diagram of a portion of a carrier-wave telephone system comprising a signal receiver according to the invention.

In the system shown in the drawing, in which the signals are transmitted by the emission of a carrier wave which issuppressed during the transmission of conversation, the conversation to be transmitted is received through the lines I and is supplied in the usual manner through a differential transformer 2 with a diagrammatically-in; dicated balancing network 3, to a modulator 4.

' The speech currents are modulated by modulator 4 on a carrier wave of frequency f which is produced by a high-frequency generator .5.,

Modulator l is designed so that the carrier wave produced by generator 5 is suppressed, and

,that there will be only two side-bands. After passing through a band pass iiterIS and conductors I4 and I5 one o f these side-bands is suppressed in-a band-pass iilter 6 and supplied to a transmission line 1. Additional similar transmission c'hannels may of course be connected to line 1.

.To provide for the signaling, a relay 8 is provided. This relay has its coil connected through a single-pole switch 9 to a suitable voltage supply (not shown) and has two single-throw, normally-open armatures III and II adapted to engage fixed contacts I2 and I3 respectively. 'Ihe heels of armatures I 0 and II are connected to the terminals of generator 5, and the contacts I2 and lI3 are connected to conductors I4 and I5. Thus, by closing and opening switch 3, calling or selective impulses supplied by generator 5 are impressed upon the line 1.

The carrier wave conversation received throughy line I6 is separated from that of the plified by a triode 2| and the amplified currentV after being transformed by a transformer 22, is

supplied as low-frequencyspeech current to line I. Controlled by the anode current ofl tube 2| is a relay 31 having a single-throw normally-closed l armature 38 which controls the signal circuit 39.

, The signal impulses of the carrier frequency f1, which come in over line I6, are supplied through conductors 23 and 2l and a transformer 25, to a triode 26. Ihe grid circuit of tube 26 comprises a condenser 214shunted by a leakage resistance 28, and therefore this tube acts as an amplitude limiter since in the presence of grid currents the grid bias is shifted, in accordancewith the maximum amplitude of the oscillations supplied to the grid circuit (thus also signal currents orl conversation side-band), toward the negative to such an extent as -to keep the maximum anode current substantially constant.

Coupled to the anode circuit of tube through a transformer 32 is a mixing circuit comprising av rectiiier 29, an'auxiliary high-'frequency generator 30 ofa frequency f2, and the primary winding of a transformer 3l. Connected across the secondary winding of transformer 3| is an oscilla' tory circuit comprising a condenser 33 and an inductance coil 3|, which circuit is tuned toa frequency fa equal to the difference between the frequencies of generator 30 and I9, i; e. fa=faf1.

The voltage set up across inductance coil S6 is rectiied by a rectifier 35 and applied across a resistance 36 in the grid circuit of tube 2l. The voltage drop produced in the grid circuit of tube 2| is in such a direction that the anode current of the tube is greatly suppressed or reduced to zero. As a result, relay 31 is deenergized and a signal impulse is produced. A

When signal currents are received there is no conversation side-band, and thus the amplitude of the signal current in the anode circuit of the limiter tube 26 correspondsfto the above-mentioned constant value of the anode current. 'I'his current is suflicient to cause relay 31 to pick up armature 38 and interrupt the signal circuit 39.

During the reception of the conversation sideband, which is composed of a plurality of frequencies, the amplitude of each of the separate frequencies in the anode circuit of the limiter tube 26 may be a fraction of the above-mentioned constant value of' the anode current. A disturbing side-band frequency consequently can produce only an anode current which is considerably smaller than the anode current which occurs in -3 aeraesa the reception of the signal current. As a result, the possibility of false responses in the signal circuit is eliminated, or at least greatly reduced.

While I have described my invention with reference to a speciiic example, I do not wish to be limited thereto but desire the appended claims to be construed as broadly as permissible in view oi' the prior art.

' What I claim is:

A carrier wave receiving circuit comprising a main channel responsive to intelligence conveyed by suppressed carrier modulated Waves and including a demodulator, a side channel derived from the main channel at a point before the demodulator and responsive to additional signals transmitted by the transmission of the carrier wave, `a. frequency selective circuit in said side channel for filtering out the additional signals,

and amplitude limiting means in said side chan- 

